1. Field of the Invention
The present invention relates to a resonating cavity, and more particularly, to a resonator system that is independent of temperature variation.
2. Background of the Invention
In the past few years, planar lightwave circuit (PLC) technologies have been widely used in several devices, such as optical filters, optical modulators and optical sensors, and so on. As the PLC technologies have been rapidly developed, the corresponding passive and active devices have also been rapidly developed. For example, as the requirements of communication capacity get higher and higher in DWDM (dense wavelength division multiplexing) system and signal wavelengths need to be set near the minimum transmission lost point of optical fibers as far as possible, the distances of each signal wavelength get narrower and narrower. Thus, it is one of the very important topics in developing semiconductor optical devices on silicon substrate for frequency selectivity. Ring filters keep the signals selectivity and are widely used in wavelength filtering applications for its characteristic of high Q value. Therefore, ring filters are important devices in optical signals processing.
FIG. 1 is a perspective drawing of a traditional resonator system. The resonator system 10 is consisted of a resonating cavity device 11 and a light source 12. The resonating cavity device 11 comprises a waveguide 13, a ring resonator (electro-optic modulator) 14 and a modulating control unit 15. The light source 12 provides an optical wave (an optical signal) with a particular wavelength entering the resonating cavity device 11 from the input end 131 of the waveguide 13 and outputting from the output end 132 of the waveguide 13. The operation principle of the ring resonator (electro-optic modulator) 14 is similar to the Fabry-Perot resonator. However, the ring resonator 14 can achieve the function of self-feedback without a reflection surface. Assuming the ring resonator (electro-optic modulator) 14 has a circumference of L and a power attenuation constant of α, it generates an attenuation volume of e−αL and a phase conversion after the optical wave being coupled to the ring resonator (electro-optic modulator) 14 and going around and back to the original coupling point in the ring resonator (electro-optic modulator) 14. The phase conversion is associated with the circumference of L and the wavelength of the inputting optical wave of the ring resonator (electro-optic modulator) 14. By the principle, parts energy of the optical wave will be transferred to the ring resonator (electro-optic modulator) 14 after the optical wave entering the resonating cavity device 11 from the input end 131 of the waveguide 13. The optical wave is continuously resonating back and forth in the ring resonator (electro-optic modulator) 14 and coupling back to the waveguide 13 and outputting from the output end 132 of the waveguide 13. Therefore, the high Q value will be achieved. OFF-state of the modulating control unit 15 is used for the ring resonator (electro-optic modulator) 14 operating in a normal mode. After receiving a voltage signal or a current signal, the ON-state modulating control unit 15 fine-tunes the peak value of the output signal of the output end 132.
Generally speaking, the ring resonator (electro-optic modulator) 14 is sensitive to the operating wavelength of the optical wave and the variation of ambient temperature. Accordingly, when the ambient temperature changes, the electro-optic characteristic of the ring resonator (electro-optic modulator) 14 will be changed extremely.
FIG. 2 is an output spectrum drawing of the traditional resonator system shown in FIG. 1. W10 and W11 are output spectrums of the modulating control unit 15 switching between ON-state and OFF-state in a certain ambient temperature condition, respectively; W20 and W21 are output spectrums of the modulating control unit 15 switching between ON-state and OFF-state in a varying ambient temperature condition, respectively. IL is the insertion loss of the ring resonator 14, and ER is the extinction ratio of the ring resonator 14. From FIG. 2, it is easy to realize that the insertion loss of the ring resonator 14 is less affected by the temperature variation when the operation wavelength is a light source with single wavelength (the range of wavelength ±0.1 nm being a single wavelength). Nevertheless, the extinction ratio of the ring resonator 14 is much affected by the temperature variation. As the temperature changes, the operating wavelength will follow the temperature variation, the insertion loss and extinction ratio will be deteriorated. For overcoming the problems above, ordinarily, it adds a temperature controller (not shown) in the resonating cavity device 11 for keeping the temperature inside the resonating cavity device 11 or decreasing the amount of temperature variation inside the resonating cavity device 11. Thus, the operation wavelength will be almost kept in a constant, so as to decrease the insertion loss and extinction ratio deterioration of the ring resonator 14.
Although the insertion loss and extinction ratio deterioration may be improved by adding the temperature controller such as the TEC (thermoelectric controller), but the manufacture cost, power consumption and device sizes are not benefit for use by using such active control method.
It is desirable, therefore, to provide a resonator system that is independent of temperature variation without adding too much manufacture cost, power consumption and device sizes.